Geo: Add setting for ActionCable allowed origins

What does this MR do?

This add a new setting to enable admins to setup allowed origins for websocket requests through ActionCable in order to ensure communication between secondary and primary Geo sites via websockets works. This fixes a bug where Duo is not working from secondary sites.

Related issues

• bug issue: Websocket requests (impacting Classic Duo Chat)... (gitlab-org/gitlab#579074)
• Rails MR: Geo: Add setting for ActionCable allowed origins (gitlab-org/gitlab!213206 - merged)
• Omnibus MR: Geo: Add setting for ActionCable allowed origins (gitlab-org/omnibus-gitlab!8910 - merged)

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• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
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